Method for arranging a packing in a burner and burner basket for a burner

ABSTRACT

A method for disposing a bed comprising particles in a burner through which a gas can flow, more particularly in a burner basket of an ammonia oxidation burner, where the particles are disposed such that the bed has a greater flow resistance in an edge region of the burner than in an inner region of the burner. Further, a burner basket for a burner may have a bed comprising particles, wherein the particles are disposed such that the bed has a greater flow resistance in an edge region of the burner basket than in an inner region of the burner basket.

PRIOR ART

The present invention relates to a method for arranging a bed consistingof particles in a burner through which a gas can flow, more particularlyin a burner basket of an ammonia oxidation burner. The invention furtherrelates to a burner basket for a burner, more particularly for anammonia oxidation burner, having a bed consisting of particles.

Burners of this kind are used as ammonia oxidation burners in thesynthesis of nitric acid, for example. In that case ammonia (NH₃) andoxygen (O₂) are reacted catalytically to form nitrogen monoxide (NO) andwater (H₂O) in the ammonia oxidation burner. The NO obtained is thenused further for preparing nitric acid.

Catalysts used in ammonia oxidation burners are customarilyplatinum/rhodium gauzes, which are placed on a bed consisting ofparticles. The particles generally are designed as packing elements madeof stoneware, glass, porcelain or stainless steel, and are introducedinto a burner basket arranged within the burner. The burner basketcustomarily has a gas-permeable bottom plate, allowing the NH₃introduced into the burner to flow through the burner basket and throughthe bed.

The oxidation of ammonia in the burner requires an operating temperatureof about 890° C. at a pressure of about 10 bar. Owing to the hightemperature, in operation of the ammonia oxidation burner, the burnerbasket undergoes expansion. An observation here is that the bottom plateof the burner basket expands with a delay in comparison to the sidewalls of the burner basket. On repeated start-up and run-down of theammonia oxidation burner, these differences in expansion characteristicsbetween burner basket side walls and bottom plate result in developmentof cavities and cracks in the bed in the edge region of the burnerbasket. This destruction of the bed structure reduces the flowresistance for the permeating stream of NH₃, and the catalyst gauzes areno longer supported uniformly by the bed. These phenomena give rise to aloss of combustion efficiency and to ammonia slip.

DISCLOSURE OF THE INVENTION

The object of the present invention is to raise the combustionefficiency and reduce the ammonia slip.

The object is achieved by a method for arranging a bed consisting ofparticles in a burner through which a gas can flow, more particularly ina burner basket of an ammonia oxidation burner, the particles beingarranged such that the bed has a greater flow resistance in an edgeregion of the burner than in an inner region of the burner.

Proposed further for achieving the object is a burner basket for aburner, more particularly for an ammonia oxidation burner, having a bedconsisting of particles, the particles being arranged such that the bedhas a greater flow resistance in one edge region of the burner basketthan in an inner region of the burner basket.

The arrangement of the particles is selected such that the bed has agreater flow resistance relative to the interior in the burner edgeregion that is susceptible to cavitation and/or gapping. The permeatinggas is guided increasingly through the inner region of the reactor,thereby reducing the permeating gas loading on cavities present in theedge region, consequently reducing the development and expansion ofcavities and gaps in the edge region. The destruction of thebulk-materials structure because of the difference in expansioncharacteristics between side walls and bottom plate, as a result oftemperature fluctuations, is limited, and so there is an increase incombustion efficiency and reduction in ammonia slip.

The bed preferably has a greater bulk density in the edge region than inthe inner region. As a result of the greater edge region bulk density,i.e., the greater mass of particles per unit volume in the edge region,the free space needed for the permeation of the gas between theparticles in the edge region is reduced and the flow resistance in theedge region is increased. The higher bulk density contributes torestricting the freedom of movement of the particles in the edge region,thereby reducing the formation of cavities and/or gaps because of theexpansion of the burner basket. In the inner region the bulk density ispreferably set such that it is lower than in the edge region.

In one advantageous refinement, the bed comprises small particles andlarge particles, the small particles having a smaller diameter than thelarge particles. Through the use of particles of different diameters itis possible to adjust the flow resistance of the bed. It is possible toform regions which have essentially small particles, in order to set ahigh flow resistance, and regions which have essentially largeparticles, in order to set a low flow resistance. Furthermore, the largeparticles and small particles can be mixed in order to adjust the flowresistance.

In this context it has emerged as being particularly advantageous if thesmall particles have a diameter in the range from 1 mm to 10 mm,preferably in the range from 2 mm to 5 mm. A bed of small particles withdiameters in the stated range has a high flow resistance.

The large particles preferably have a diameter in the range from 5 mm to50 mm, preferably in the range from 10 mm to 40 mm, more preferably inthe range from 20 mm to 30 mm. A bed of large particles having diametersin the stated range has a low flow resistance.

The ratio of the diameter of the small particles to the diameter of thelarge particles is preferably in the range from 1/50 to 1, morepreferably in the range from 1/50 to 1/25.

In one preferred refinement, more small particles than large particlesare arranged in the edge region and/or more large particles than smallparticles are arranged in the inner region. The preponderance of smallparticles in the edge region raises the bulk density and the flowresistance in the edge region. The preponderance of large particles inthe inner region reduces the flow resistance in the inner region. Withparticular preference, small particles are arranged substantially in theedge region and/or large particles are arranged substantially in theinner region, so producing a maximum flow resistance in the edge regionand/or a minimum flow resistance in the inner region.

According to an alternative refinement, more small particles than largeparticles are arranged in the edge region and in the inner region twolayers are arranged, the lower layer having more small particles thanlarge particles and the upper layer having more large particles thansmall particles. An arrangement of this kind for the particles allowsthe stability of the bed structure to be improved further.

In a further alternative refinement, in the edge region a mixture ofsmall particles and large particles is arranged, and so the flowresistance in the edge region is set through the mixing ratio of thesmall particles and large particles. The mixing ratio can be set bymeans of a mixing apparatus to which small particles and large particlesare supplied separately. The number of large particles and smallparticles in the mixture is preferably substantially the same.

Preference is given to the arrangement in the edge region of mutuallysuperposed layers of large particles and of small particles. Largeparticles and small particles may be introduced in alternation into theedge region.

The width of the edge region preferably has a value in the range from 1%to 6% of the diameter of the burner and/or of the diameter of the burnerbasket. The width of the edge region is advantageously in the range from5 cm to 30 cm, preferably in the range from 10 cm to 20 cm.

According to one advantageous refinement, a gas-permeable separationmaterial to which the bed is applied is arranged on a bottom plate ofthe burner or of the burner basket. The gas-permeable separationmaterial prevents the particles of the bed slipping through any openingsin the bottom plate of the burner basket and/or through any gaps betweenthe bottom plate and the side wall of the burner basket.

In one preferred refinement, a separating device is introduced into theburner or the burner basket and separates the edge region from the innerregion. As a result of the separating device it is possible to preventunwanted mixing of the edge region particles with the inner regionparticles. The separating device is preferably introduced into theburner or the burner basket before the particles of the bed are arrangedin the edge region and/or in the inner region. The separating device maybe removed from the burner or the burner basket after the bed has beenintroduced.

It has proven preferable for a gas-permeable separation material, moreparticularly a mesh, to be introduced between the edge region and theinner region. As a result of the gas-permeable separation material,mixing of the particles introduced into the edge region and the innerregion can be prevented. The gas-permeable separation material isarranged preferably in the burner, more particularly in the burnerbasket, before the bed is introduced. With particular preference thedesign of the separation material is such that it is not permeable forsmall particles and large particles. The gas-permeable separationmaterial can be introduced loose into the burner, more particularly intothe burner basket, or anchored, for example to the bottom plate of theburner basket and/or to a separation material lying on the bottom plate.It is preferred, furthermore, for the gas-permeable separation materialto be elastic, and so able to deform when the particles move. Thegas-permeable separation material which is introduced between the edgeregion and the inner region may be formed from the same material as thegas-permeable separation material which is arranged on the bottom plate.The separation material separating the edge region and the inner regionmay remain in the burner during operation of the burner.

It is advantageous if the edge region has a rectangular, moreparticularly square, cross section. As a result of the rectangular, moreparticularly square, cross section, the stability of the structure ofthe bed in the edge region can be increased and the formation ofdepressions, cavities and/or gaps can be counteracted.

In an alternative refinement, the edge region has a trapezoidal crosssection. A trapezoidal edge region is an advantage in those burnersand/or burner baskets which are conical in design. The trapezium may beformed with an upward or downward taper.

The particles of the bed may be designed as packing elements, forexample as Raschig rings, Pall rings, Berl, Interlox or Torus saddlesand/or Interpack bodies. The material of the packing elements ispreferably stoneware, porcelain, glass or stainless steel. Alternativelyor additionally, the particles of the bed may have a catalyst. The bedaccordingly may form a secondary catalyst, which increases theefficiency of the catalytic reaction. For example, the particles may bedesigned as packing elements which are impregnated with a catalyst, oras particles formed from a catalyst-comprising material. It is possibleto use a mixture of particles formed of a non-catalyst-comprisingmaterial and particles formed of a catalyst-comprising material.

The advantageous features described above can be used in the case bothof the method of the invention and of the burner basket of theinvention, alone or in combination.

Further details, features and advantages of the invention are apparentfrom the drawings, and also from the description below of preferredembodiments with reference to the drawings. These drawings illustratemerely exemplary embodiments of the invention, which do not restrict theinventive concept.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a first refinement of a burner basket of the invention in aschematic sectional representation.

FIG. 2 shows a second refinement of a burner basket of the invention ina schematic sectional representation.

FIG. 3 shows a third refinement of a burner basket of the invention in aschematic sectional representation.

FIG. 4 shows a fourth refinement of a burner basket of the invention ina schematic sectional representation.

FIG. 5 shows a fifth refinement of a burner basket of the invention in aschematic sectional representation.

FIG. 6 shows a sixth refinement of a burner basket of the invention in aschematic sectional representation.

FIG. 7 shows a seventh refinement of a burner basket of the invention ina schematic sectional representation.

FIGS. 8a-c show a burner basket as in FIG. 1 in different states, toillustrate a first refinement of the method of the invention forarranging the bed.

FIGS. 9a-c show a burner basket as in FIG. 2 in different states, toillustrate a second refinement of the method of the invention forarranging the bed.

EMBODIMENTS OF THE INVENTION

In the various figures, identical parts are always provided with thesame reference numerals, and are therefore in general only identified ormentioned once in each case as well. The drawings are schematicrepresentations which serve to illustrate fundamental relationships. Therepresentations are not true to scale and nor do they correctlyreproduce the size relationships described.

FIG. 1 shows a burner basket 1 of a burner 10 formed as an ammoniaoxidation burner, by means of which ammonia and oxygen are reactedcatalytically to give nitrogen monoxide and water. The burner basket 1has a substantially conical shape and, in operation of the burner 10, itis arranged in the interior of the burner 10, and so can be traversed byflows of ammonia and oxygen. The burner basket 1 is formed from agas-permeable bottom plate 3 and side walls 2. In the case of thepresent exemplary embodiment, the gas-permeable bottom plate 3 and theside walls 2 are fixed independently of one another in the burner andare not directly joined to one another. Accordingly there is a gapbetween the gas-permeable bottom plate 3 and the side walls 2. Arrangedabove the bottom plate 3 is a gas-permeable separation material 4, whichallows the passage of ammonia and oxygen and at the same time preventsparticles falling through the gap between bottom plate 3 and side walls2 or through the bottom plate 3.

Situated within the burner basket 1 is a bed 5 of particles which are inthe form of packing elements 8, 9. In the figures, the packing elements8, 9 are shown for simplification as substantially spherical particles,although particles of any predetermined form—as Raschig rings, Pallrings, Berl, Interlox or Torus saddles and/or Interpack bodies, forexample, may constitute these elements, in deviation from therepresentation in the figures. The material of the packing elements ispreferably stoneware, porcelain, glass or stainless steel. Arrangedabove the bed 5, not shown in the figures, may be a catalyst gauze, suchas a platinum/rhodium catalyst gauze, for example. The particles mayoptionally have a catalyst material, and so the catalytic activity isenhanced.

In order to increase the combustion efficiency and to reduce the ammoniaslip, the particles 8, 9 are arranged in such a way that the bed 5 has agreater flow resistance in an edge region 6 of the burner basket 1 thanin an inner region 7 of the burner basket 1. As a consequence of theincreased flow resistance in the edge region 6, the mixture of ammoniaand oxygen is guided to an increased extent through the inner region 6of the burner basket 1. The bed 5 has a greater bulk density in the edgeregion 6 than in the inner region 7. The higher bulk density in the edgeregion 6 contributes to restricting the freedom of movement of theparticles 8 in the edge region 6, thereby reducing the formation ofcavities and/or gaps because of thermally induced expansions of thebottom plate 3 and/or of the side walls 2.

As is also apparent from the representation in FIG. 1, the bed 5comprises small particles 8 and large particles 9, the small particles 8being smaller in form than the large particles 9. The diameter of thesmall particles 8 is in the range from 1 mm to 10 mm and is smaller thanthe diameter of the large particles 9, which is in the range from 5 mmto 50 mm.

Substantially small particles 8 are arranged in the edge region 6 of theburner basket 1, while substantially large particles are arranged in theinner region 7. Accordingly in the edge region 6 there is apreponderance of small particles and in the inner region 7 there is apreponderance of large particles. The edge region 6 has a width which isbetween 1% and 6% of the diameter of the burner.

FIG. 2 shows a second exemplary embodiment of a burner basket 1 of theinvention. Fundamentally, the burner basket 1 has a construction similarto that of the burner basket of the first exemplary embodiment, and sowhat was said there is also valid for the second exemplary embodiment.In contrast to the burner basket 1 of the first exemplary embodiment,the burner basket 1 according to FIG. 2 additionally has a gas-permeableseparation material 11 which is arranged between the edge region 6 andthe inner region 7. The gas-permeable separation material 11 is designedas an elastic mesh which is able to deform on expansion of the burnerbasket 1, as a result of the heating thereof, thereby removing a risk ofdamage to the gas-permeable separation material 11 as a result of themovement of the particles 8, 9.

FIG. 3 shows a third exemplary embodiment of a burner basket 1 accordingto the invention. The burner basket 1 of the third exemplary embodimentcorresponds to the burner basket 1 of the first exemplary embodiment,with the difference that the arrangement of the particles in the innerregion 7 of the burner basket 1 is different. According to FIG. 3, thereare two layers arranged in the inner region 7, with the lower layerhaving more small particles 8 than large particles 9 and the upper layerhaving more large particles 9 than small particles 8. In the edge region6 there are more small particles 8 than large particles 9 arranged. As aresult, the stability of the bed 5 is improved relative to the bed 5 ofthe first exemplary embodiment.

The representation in FIG. 4 shows a fourth exemplary embodiment of aburner basket 1 of the invention. In comparison to the precedingexemplary embodiments, the basic form of the burner basket 1 accordingto FIG. 4 is cylindrical. The side walls 2 are arranged substantially ata right angle to the bottom plate 3. Moreover, the side walls 2 arejoined directly to the bottom plate 3.

Since the side walls 2 run substantially vertically, the edge region 6has a rectangular, more particularly square, cross section. Arranged inthe edge region 6 is a mixture of small particles 8 and large particles9. The particles 8, 9 of the bed 5 are arranged in layers in the edgeregion 6, each layer having essentially small particles 8 or largeparticles 9.

FIG. 5 shows a fifth exemplary embodiment of a burner basket 1. Theburner basket 1 corresponds essentially to the burner basket 1 shown inFIG. 3, with the difference that a separation material 11 in the form ofa separation mesh is introduced in order to separate the large particles9 from the small particles 8. The separation material 11 is arrangedbetween a lower layer, which consists of small particles 8, and an upperlayer, which consists of large particles 9.

FIG. 6 shows a sixth exemplary embodiment of a burner basket 1, whichcorresponds essentially to the burner basket 1 shown in FIG. 4. For theseparation of the large particles 9 from the small particles 8, aplurality of separation materials 11 in the form of separation meshesare introduced into the burner basket 1. The separation meshes arearranged substantially horizontally, and separate a layer consisting oflarge particles 9 from the bordering layers, which comprise largeparticles 9 and small particles 8.

The representation in FIG. 7 shows a seventh exemplary embodiment of aburner basket 1, which has a bed 5 having in the edge region 6 anincreased flow resistance relative to the inner region 7. For thispurpose, a mixture of small particles 8 and large particles 9 isintroduced in the burner basket 1, there being fewer large particles 9per unit volume arranged in the edge region 6 than in the inner region7, so producing a mixture of higher bulk density in the edge region 6.The number of small particles 8 per unit volume is greater in the edgeregion 6 than in the inner region 7.

The small particles 8 are formed of a catalyst material, while the largeparticles 9 consist of ceramic. The large particles 9 are designed asRaschig rings. The size selected for the Raschig rings is such that thesmall particles 8 are able to penetrate the cylindrical cavity formed bythe Rashig rings. This brings with it the advantage that the smallparticles 8 are held by the large particles 9 in the form of Raschigrings in the edge region 6, thereby reducing the risk of the blowing ofthe small particles 8 from the edge region 6 in the direction of theinner region 7. Arranged between the edge region 6 and the inner region7 there are, additionally, separation meshes 11 made from agas-permeable material, so making it more difficult for unwantedmigration of the small particles 8 from the edge region 6 into the innerregion 7 to take place.

A first refinement of the method of the invention for arranging a bed 5in a burner 10 through which a flow of gas may pass will be elucidatedbelow with reference to the representations in FIG. 8.

As shown in FIG. 8a , a gas-permeable separation material 4, for examplein the form of a mesh, is first of all arranged on the bottom plate 3.The separation material 4 may be arranged in such a way that itprotrudes beyond the bottom plate 3 at the sides and bears against theside walls 2.

In a further step, which is shown in FIG. 8b , a separating device 12 isintroduced into the burner basket 1. The separating device has at leastone separating wall which separates the inner region 7 from the outerregion 6 of the burner basket 1. The separating device 12 may bedesigned, for example, in the manner of a cylindrical pipe.

When the separating device 12 has been introduced into the burner 10,the bed 5 is introduced into the burner basket 1 of the burner 10. As isapparent from FIG. 8c , the particles 8, 9 of the bed 5 are arranged inthis case such that the bed 5 has a greater flow resistance in the edgeregion 6 of the burner basket 1 than in an inner region 7 of the burnerbasket 1. The edge region 6 is filled with more small particles 8 thanlarge particles 9. In the inner region 7 there are more large particles9 than small particles 8 introduced.

After the introduction of the bed 5 into the burner basket 1, theseparating device 12 is removed from the burner basket 1. The particles8, 9 fill the space vacated by the separating device 12, and anarrangement is produced as shown in FIG. 1.

Lastly it is possible for a catalyst gauze to be placed onto the bed 5.

A further refinement of the method of the invention is described belowwith reference to the representation in FIG. 9.

As shown in FIG. 9a , a gas-permeable separation material 4, in the formof a mesh, for example, is first of all arranged on the bottom plate 3.The separation material 4 may be arranged in such a way that itprotrudes laterally beyond the bottom plate 3 and bears against the sidewalls 2.

As shown in FIG. 9b , in a subsequent method step, at least onegas-permeable separation material 11 is arranged in the region betweenthe edge region 6 and the inner region 7 of the burner basket 1. Thegas-permeable separation material 11 is preferably joined to thegas-permeable separation material 4 lying on the bottom plate 3.

When the gas-permeable separation material 11 has been introduced intothe burner basket 1, the bed 5 is introduced into the burner basket 1 ofthe burner 10. As is apparent from FIG. 9c , the particles 8, 9 of thebed 5 are arranged in this case such that the bed 5 has a greater flowresistance in the edge region 6 of the burner basket 1 than in an innerregion 7 of the burner basket 1. In the edge region 6, there are moresmall particles 8 introduced than large particles 9. In the inner region7, there are more large particles 9 introduced than small particles 8.

Lastly a catalyst gauze can be placed onto the bed 5.

With the above-described method for arranging a bed 5, consisting ofparticles 8, 9, in a burner 10 through which a flow of gas may pass,more particularly in a burner basket 1 of an ammonia oxidation burner,the particles 8, 9 are arranged in such a way that the bed 5 has agreater flow resistance in an edge region 6 of the burner 10, than in aninner region 7 of the burner 10. As a result of this, the combustionefficiency is increased and the ammonia slip is reduced.

LIST OF REFERENCE NUMERALS

-   1 Burner basket-   2 Side wall-   3 Bottom plate-   4 Separation material-   5 Bed-   6 Edge region-   7 Inner region-   8 Small particles-   9 Large particles-   10 Burner-   11 Separation material-   12 Separating device

1.-16. (canceled)
 17. A method for disposing a bed comprising particles in a burner through which a gas can flow, the method comprising disposing the particles such that a flow resistance of the bed is greater at an edge region of the burner than at an inner region of the burner.
 18. The method of claim 17 comprising disposing the bed comprising particles in a burner basket of the burner, wherein the burner is an ammonia oxidation burner.
 19. The method of claim 17 wherein the bed has a greater bulk density in the edge region than in the inner region.
 20. The method of claim 17 wherein the bed comprises small particles and large particles, wherein the small particles have a smaller diameter than the large particles.
 21. The method of claim 20 wherein the small particles have a diameter in a range from 1 mm to 10 mm.
 22. The method of claim 21 wherein the large particles have a diameter in a range from 5 mm to 50 mm.
 23. The method of claim 22 wherein more of the small particles than the large particles are disposed in the edge region of the burner, wherein more of the large particles than the small particles are disposed in the inner region of the burner.
 24. The method of claim 22 wherein more of the small particles than the large particles are disposed in the edge region, wherein two layers of particles are disposed in the inner region, wherein a lower layer of the two layers has more of the small particles than the large particles and an upper layer of the two layers has more of the large particles than the small particles.
 25. The method of claim 22 comprising disposing a mixture of the small particles and the large particles in the edge region.
 26. The method of claim 22 comprising disposing mutually superposed layers of the large particles and the small particles in the edge region.
 27. The method of claim 22 wherein a width of the edge region of the burner has a value in a range from 1% to 6% of at least one of a diameter of the burner or a diameter of a burner basket of the burner.
 28. The method of claim 17 wherein a gas-permeable separation material to which the bed is applied is disposed on a bottom plate of the burner.
 29. The method of claim 17 further comprising introducing a separating device into the burner that separates the edge region from the inner region.
 30. The method of claim 17 further comprising introducing a gas-permeable separation material between the edge region and the inner region.
 31. The method of claim 17 wherein the edge region has a rectangular cross section or a trapezoidal cross section.
 32. The method of claim 17 wherein the particles of the bed at least one of have a catalyst, or are configured as packing elements.
 33. A burner basket for a burner, the burner basket comprising a bed of particles disposed such that a flow resistance of the bed is greater in an edge region of the burner basket than in an inner region of the burner basket.
 34. The burner basket of claim 33 wherein the burner basket is configured for an ammonia oxidation burner, wherein a gas-permeable separation material to which the bed is applied is disposed on a bottom plate of the burner basket.
 35. The burner basket of claim 33 wherein the bed comprises small particles and large particles, wherein the small particles have a smaller diameter than the large particles, wherein the small particles have a diameter in a range from 1 mm to 10 mm, wherein the large particles have a diameter in a range from 5 mm to 50 mm.
 36. The burner basket of claim 35 wherein more of the small particles than the large particles are disposed in the edge region, wherein two layers of particles are disposed in the inner region, wherein a lower layer of the two layers has more of the small particles than the large particles and an upper layer of the two layers has more of the large particles than the small particles. 